[boomboom_shaka]

I have a Maytag Microwave (model MMV5165AAS) about 5 years old. It seemed to work fine all this time until today I noticed a problem.
When I close the door, the light stays on and some motor sound turns on (probably the fan inside). I put a cup with water for 1-2 mins, the turntable moves as normal, but the water didn't get warm (meaning it's just the light which is on and the sound, but not the microwave operation). When I use the Cook operations/controls, the turntable moves as normal, light is on, sound is there AND the water gets warm (meaning the microwave works).
However, it is really annoying because when I close the door, the light is supposed to turn off with no motor (fan) sound.
I first thought may be it is the vent which is turning on because of some heat (even though I know I didn't cook anything underneath).

Could someone please help me diagnose the problem and then let me know how to fix it.

Thanks in Advance.